Okay, when I first saw the title I confess I nearly blew Shiraz all over the screen. "Nah, what really matters is the glint in her eye." But now that I've winched my mind out of the gutter ...

I think the absolute minimum size for a wilderness cup is a pint (500 ml). Especially in winter, when you may be melting snow and fire is not always easy to come by.

If you have two smaller steel cups (or tin cans) you can alternate between them, with one melting snow or boiling runoff, and the other cooling off to fill the canteen or make tea/soup. A very practical arrangement given the need for ongoing hydration in winter.

I have a couple of those military canteen cups and they are outstanding, but they are also very heavy and bulky. While sturdy and effective, they are also rather tempting to leave at home, which doesn't help you at all.

In the past I have used enamel-on-steel cups, which are a bit lighter but just as bulky. Though I had one with just enough room for a Svea and several packets of Earl Grey -- perfect for long daytrips.

As for Sierra cups, the oversize version (16 oz / 500 ml) is quite useful, though it's hard to fill them more than 3/4 full over a fire. You can wire them to a longer stick for a handle. They pack up very compactly. And don't forget their classic function -- slipping the handle of one or two under your belt as you "just stroll down that trail for an hour to see if there's anything interesting down there." Nice slim profiles; you won't even notice them until you need them.